Facile synthesis of template-free 3D-connected hierarchically porous CaCu3Ti4O12 monolith by a green aqueous sol-gel method

Calcium copper titanate (CCTO) is a novel functional material renowned for its exceptional dielectric properties and recent application as a photocatalyst. This study employs a green aqueous facile sol-gel technique to synthesize template-free, 3D-connected hierarchically porous monolithic CCTOs under two gelation and aging conditions (30 °C and 40 °C). Both samples exhibited identical thermal, crystalline, compositional, optical, and bandgap characteristics, with intrinsic direct and indirect band gaps of 1.99 eV and 1.71 eV, respectively. However, the porosity differed: the 30 °C sample had a higher macropore volume, while the 40 °C sample had a higher meso-/micropore volume, resulting in BET surface areas of 127 m²/g and 53 m²/g, respectively. Despite high calcination at 1100 °C (> 100 nm particle size), which reduced surface areas to 0.4 m²/g and 0.5 m²/g, the pore structures remained. These one-step, template-free hierarchically porous CCTO materials show significant potential in catalysis, offering advantages such as efficient mass transport within macropores, high active surface area within meso-/micropores, reduced diffusional limitations, improved catalyst recovery due to their monolithic morphology, and visible region light absorbance for photocatalysis. The research shows the mesopore/surface area can be improved via lower-temperature calcination (e.g. 800 °C; 40 nm particle size) with extended duration, or alternative techniques like microwave calcination.